The Data Horizon is a tool, used to define (limit) the time span for the marked data used for backtestesting.
Each Data Source has its own Data Horizon settings. Here is where you choose which Data Source you want to change.
FSB Pro can use data from shorter periods data files. This setting tells FSB Pro how many shorter period bars it may load. See more about that in Intrabar Statistics
If the Generator or the Optimizer is working with this data, they will not reload the data upon changing the settings. This happens because the Generator and the Optimizer copy the data for their own use when they start working. However if you are editing a strategy, for example, and click Reload Corresponding Files this will reload the data for the backtest.